Similarity between this earth and the other Planets

We may observe a very great [similarity] between this earth which we inhabit, and the other planets, Saturn, Jupiter, Mars, Venus, and Mercury. They all revolve around the sun, as the earth does, although at different distances and in different periods. They borrow all their light from the sun, as the earth does. Several of them are known to revolve around their axis like the earth, and, by that means, must have a like succession of day and night. Some of them have moons that serve to give them light in the absence of the sun, as our moon does to us. They are all, in their motions, subject to the same law of gravitation, as the earth is. From all this similarity it is not unreasonable to think that those planets may, like our earth, be the habitation of various orders of living creatures. There is some probability in this conclusion from analogy.6

The argument here seems to be that some other planet probably supports life, because Earth does and other planets are similar to Earth in revolving around the sun and around an axis, getting light from the sun, and so on. What makes certain analogies relevant is not, of course, that the motion of Earth is explained by the presence of life here. Rather, certain features of Earth explain why Earth is habitable. The argument suggests that the best explanation of why there is life on our planet is that certain conditions make life possible. That generalization can then be used to support the conclusion that other planets with the same conditions probably support life as well.